If its your knee suddenly locking up then that sounds like a possible cartiledge tear maybe. Can you fully straighten your leg at the knee or is that impossible? If you cant straighten it fully and its the knee locking up then that may be indicative of the cartiledge problem. Other than that nothing else spring to mind lynnelot. Maybe someone else will have ideas.

Have you ever been checked to see if you have a disc problem? They asked me about this symptom when they discovered that I have a bulged and torn disc (fortunately, I don't have this problem).

It could be a disc problem causing a nerve problem that is causing both your back pain and your leg to go out. I think you should have it checked out. You should definitely call your doctor if you are having your legs collapse causing you to fall. That is not something to simply tolerate.

I have fallen several times due to the fact my legs just go numb and give out. The last time I felt really smart and was weedeating the bank behind the house. The next thing I knew I was face down on the ground with the weed eater right beside me. That was and will be the last day I weed eat. I do have disc problems along with scolosis and AS so I have a very complicated back. I have not gone to the dr because they will only want to do more tests and I will get the same answers. But if you have not seen the dr please make an appointment.

fYI, my pain mangement doctor was the one to find my disc problems. He knew instinctively that pain down the back of the legs is related to bad discs. My neurologist was worthless..he could not even rid me of my almost daily migraines. All he wanted to do was add ten more drugs to my ever expanding list. No thanks. Good luck!